The present invention relates to a method for determining the dip angle of the geological formations traversed by a borehole.
Determination of the dip angle of geological formations using magnetic measurements is already known, for example, from U.S. Pat. No. 3,388,323.
Other dipmeters are operated based on the measurement of the electrical resistivity of the geological formations along a plurality of generatrices of the borehole wall.
Such measurments are carried out by means of a sonde provided with electrodes which must be placed in the immediate vicinity of the geological formations.
Dipmeters are also known which are operated based on the measurement of the velocity or of the propagation time of an acoustic wave through the geological formations, along a direction substantially parallel to the axis of the drilled wellbore.
Such dipmeters are, for example, described in U.S. Pat. Nos. 3,295,100, 3,376,950 and 3,526,874.
When the borehole is filled with electrically insulating drill muds, such as oil-containing muds, the dip cannot be determined by using electrical electrodes.
Similarly, when a casing must be inserted into the borehole to prevent collapsing of its wall before all the necessary measurements have been carried out, it is no longer possible to measure the electrical resistivity of the geological formations in order to determine their dip angle, by using the above-indicated dipmeters.